An antimicrobial surface is one that presents an antimicrobial agent that inhibits or reduces the ability of microorganisms to grow. Antimicrobial agents are agents that kill microorganisms or inhibit their growth. Antimicrobial agents can be classified by the microorganisms that they act against. For example, antibacterials are used against bacteria, anti-fungals are used against fungi and anti-virals are used against viruses.
Such surfaces are desirable to prevent the spread of infection and so are desirable in healthcare settings such as hospitals, hospices, retirement homes and clinics, for example. However they are equally desirable in other settings including the home, community, transport, office environment or other public and private areas.
Whilst a material may or may not be inherently antimicrobial, the present application is directed generally to surfaces which do not possess inherent or sufficient antimicrobial properties and require a surface treatment or coating to become antimicrobial.
One area, where research has been focussed is the antimicrobial properties of copper and its alloys (brasses, bronzes, cupronickel, copper-nickel-zinc, and others). These antimicrobial materials have intrinsic properties which can destroy a wide range of microorganisms. As a result, copper and copper alloy surfaces are an effective means for preventing the growth of bacteria. Silver and zinc are also known for use in the field of antimicrobial agents.
An alternative approach is that of photocatalytically active pigments such as titanium dioxide (TiO2) or zinc oxide (ZnO) which have been used on glass, ceramic, and steel substrates for self-cleaning and antimicrobial purposes. The term “photocatalytically active pigment” means that the pigment uses the power of visible and ultraviolet light to generate oxidising agents on treated surfaces that destroy microorganisms such as bacteria, fungi and viruses on the surfaces.
For example, TiO2 reacts with light of appropriate wavelength resulting in the activation of TiO2, and creates a number of reactive oxygen species (ROS) such as hydroxyl radicals and superoxide anions after reacting with atmospheric oxygen and water. This can be explained by the following equations:TiO2+Light (hν)→Photogenerated hole (h+VB)+Electron (e−CB)  (1)Water (H2O)+h+VB→.OH+H+  (2)Oxygen (O2)+e−CB→O2.−  (3)
The hydroxyl radical as ROS is mainly responsible for the anti-microbial action, although other ROS such as singlet oxygen, hydrogen peroxide and the superoxide radical have also been reported to be involved in the process.
Titania has been used as an antimicrobial, self-cleaning, or depolluting coating on tiles, paving slabs, deodorizers, self-cleaning windows, and many more. Such an approach is described in WO2010064225-A1, in which a process for synthesising a visible light active high temperature stable anatase phase undoped titanium dioxide photocatalyst is provided comprising the step of reacting hydrated titanium dioxide with hydrogen peroxide in an aqueous solution to form a sol.
However, the processing described in WO2010064225-A1 is time consuming as it involves multiple processing steps. The sol is also unstable and is solvent based.
Furthermore, the coating in WO2010064225-A1 is more suitable for use with substrates having a high temperature stability such as ceramic tiles or roof tiles for example, which are processed above 1000° C. Glass will soften and will lose its morphological properties at a temperature typically just over 700° C. depending on the type of glass.
U.S. Pat. No. 8,551,909B describes a method of making a photocatalyst comprising a visible light activatable mesoporous titanium dioxide. The process mixes titanium isopropoxide with boiling water and the resulting solution with precipitated hydrated titanium oxide was then microwaved, filtered and dried to produce a white powder. This powder may be used as an additive for a non-transparent antimicrobial coating.
In addition, many antimicrobial coatings that are currently available require UV light for activation. UV light may not be readily available in many indoor environments where there is a need for antimicrobial coatings, for example in hospitals, clinics, offices, public transport or other community areas.
Despite developments in the field of antimicrobial coatings, there remains a need for improved antimicrobial coatings and processes for the preparation and deposition of such coatings.